In a saucepan, add Butter (3/4 cup) Guinness® Extra Stout Beer (1 cup) in a medium heat until the butter is melted. Then, add Unsweetened Cocoa Powder (3/4 cup), Caster Sugar (2 cup) and mix together. Set aside.
In a large bowl, add All-Purpose Flour (1 3/4 cup), Baking Soda (2 teaspoon), Salt (1/2 teaspoon) and whisk together. In a different jug, add Sour Cream (2/3 cup), Large Egg (2), Vanilla Extract (2 teaspoon) and mix together. Then add butter and beer mixture and mix it up. Add it in the dry ingredient and mix it together.
Pour the batter evenly in two 6 inch cake tin and bake for 40 to 45 minutes at 350 degrees F (180 degrees C).
Add Butter (1/2 cup), Powdered Confectioners Sugar (1 pound) in a stand mixer and mix it up until it's well incorporated. Then, add Bailey's® Irish Cream (3 teaspoon) and keep mixing until it becomes buttercream. Take the cakes and cut in half. Spoon the buttercream on the cake and smooth it. Form the cake to four layers. Serve and enjoy!